In the following text, the terms "inlet" and "outlet" designate the inlet and outlet ends of an adsorber in the adsorption phase; the expression "cocurrent" designates the direction of circulation of the gas in the adsorber during this adsorption phase, and the expression "countercurrent" designates the inverse direction of circulation.
In the PSA cycles applied to purification of hydrogen, the object of the pressure balancings is a reduction in the losses of the gas to be produced. Thus, in order to recover the hydrogen which is close to the outlet of the adsorber at the end of the adsorption phase, the regeneration phase includes at least one first cocurrent decompression stage by balancing the pressures with another adsorber which is undergoing a first countercurrent recompression stage. Considering, for simplification, the cycle with a single balancing step, in the course of the decompression stages which follow the first decompression by balancing, the gas extracted from the adsorber is generally used for eluting another adsorber at the low cycle pressure and is then extracted, as residual gas.
The gas still present in the adsorber at the balancing pressure can thus be considered as lost. In fact, the higher the pressure, the greater the volume of gas contained in an adsorber. Consequently, the lower the pressure of balancing, the lower the hydrogen losses, that is to say the higher the extraction yield.
A further constraint imposed on PSA installations is that they should deliver a substantially constant production rate. For this purpose, in the case where feeding the installation is carried out at constant rate (which is generally the case for hydrogen production installations), it has been proposed to take off a constant rate of product gas during the adsorbence phase and to feed this gas in countercurrent to the adsorber during the first recompression stage by balancing simultaneously with the first recompression gas. However, this method leads to a rise in the balancing pressure, and this reduces the yield as explained above.
It has also been proposed to introduce an auxiliary capacity into the production sequence with the purpose of substantially eliminating the variations in the production rate. However, this solution has the serious disadvantage of requiring an auxiliary capacity of very large dimensions if the desired pressure of use is only slightly lower than the pressure of the product gas at the outlets from the adsorbers.